In order to take off, a multi-jet commercial aircraft accelerates, by rolling on the ground, from a stopped position until it takes off from the ground.
In the course of this acceleration, the aircraft firstly reaches its minimum ground control speed VMCG, which is the minimum speed necessary for the aircraft to remain laterally controllable by the pilot, should there be a fault with of one of its jets. By continuing its acceleration, the aircraft reaches its decision speed, or critical speed V1, below which the pilot may decide to interrupt the takeoff, and above which the pilot is compelled to continue the takeoff. This critical speed V1 is necessarily greater than the minimum ground control speed VMCG but it may, in certain cases, be very close to the latter. To avoid exiting the runway should the takeoff be interrupted, the aircraft must be capable of accelerating to its critical speed V1, and then of braking to a complete stop, while traveling a distance, designated by the English expression “Accelerate-Stop Distance” or by the acronym “ASD”, which must be less than the length of the runway available. This available runway length is sometimes designated by the English expression “Accelerate-Stop Distance Available” or by the acronym “ASDA”. It may in certain cases be greater than the length of the runway itself, when the runway is followed by an extension on which the aircraft can travel or roll under exceptional conditions.
By continuing its acceleration after having exceeded the critical speed V1, the aircraft reaches its rotation speed VR. When the aircraft reaches this speed, the pilot acts on the control surfaces to make the aircraft take off.
The minimum ground control speed VMCG, the critical speed V1 and the rotation speed VR are determined, in accordance with regulatory requirements, as a function of measurements performed during trials of the aircraft and of parameters such as the maximum weight of the aircraft on takeoff (often designated by the English expression “Maximum Take Off Weight” or by the acronym “MTOW”), the thrust power of the jets of the aircraft, the length of the takeoff runway and the atmospheric conditions of the day (temperature, pressure).
When the takeoff runway is relatively short, the ASD distance must be reduced, with respect to the ASD distance used on a long runway, in order to remain less than the available runway length. This reduction in the ASD distance can be obtained by decreasing the maximum weight of the aircraft on takeoff MTOW or by decreasing the critical speed V1.
Decreasing the maximum weight on takeoff MTOW makes it necessary to reduce the quantity of fuel or to reduce the useful weight transported. This decrease affects the profitability of the flight and is therefore avoided as far as possible. One seeks on the contrary, in general, to increase this weight.
Decreasing the critical speed V1 often makes it necessary, when the takeoff runway is short, to reduce the minimum ground control speed VMCG. This decrease can be obtained by reducing the level of the thrust of the jets. Indeed, with a reduced thrust value (called by the English expression “derated thrust”), the minimum ground control speed VMCG is reduced, and the aircraft is more easily controllable in case of a fault with one of its jets. The thrust control lever for the jets of an aircraft thus comprises a control making it possible to reduce the thrust, according to a desired reduction rate.
On certain takeoff runways of short length, the maximum weight on takeoff MTOW can therefore be more significant with a reduced thrust of the jets than with the nominal thrust of the jets. However, there are still configurations, on takeoff runways of short length, in which the use of the reduced thrust of the jets is not sufficient to avoid limitations of the maximum weight on takeoff MTOW, and therefore of the performance of the aircraft on takeoff.